The relation of lipid structure to the physical states of the bulk lipid phases of the normal and diseased artery is relatively well understood, but little is known of the number and type of surface phases which can be present on cholesteryl ester rich deposits of the type associated with atherosclerosis. We have developed a model system employing lipid monolayers at the air-water interface which is useful for characterizing surface phases in equilibrium with cholesteryl ester rich bulk phases. For binary and ternary lipid mixtures containing cholesteryl esters we can determine the number of surface phases which exist at any temperature, surface pressure and composition, and the reversibility of the collapse of surface phases to more condensed or bulk phases. Comparison of the areas occupied by the molecules in mixtures with that for each compound alone yields information which is useful for defining the composition of each surface phase and possible packing arrangements of the molecules in that phase. We propose to study the surface behavior of mixtures containing cholesteryl esters as a function of the structure of the cholesteryl ester and the other lipids in mixtures. From sch a systematic study we will determine how lipid structure can influence the distribution of cholesteryl esters between bulk and surface phases and the state(s) of molecules in surface phase(s). The results will provide a background for studying cholesteryl ester hydrolysis by aortic enzymes and thereby contribute to our understanding of how lipid structure influences the removal of these esters from the arterial wall.